In Radiant, when you right-click a brush or a patch, you'll see something along the lines of: MAKE STRUCTURAL, MAKE DETAIL, MAKE WEAPON CLIP, MAKE NON-COLLIDING. What do these mean? Let me explain.
MAKE STRUCTURAL
This is your default geometry type. This has collision for players, bullets, entities and the like. Most of your brushwork / patchwork will be "structural".
This does effect umbra baking (portals) and navmesh generation.
DETAIL
This is technically the same as "make structural", however this should be used for crazy-detailed brushes or patches.
This does not effect umbra baking but does effect navmesh generation.
WEAPON CLIP
This is where things change. If you make your brush or patch a "weapon clip", that means it will become non-colliding to any player or AI, however, bullets and grenades WILL NOT pass through.
This does not effect umbra baking and navmesh generation.
NON-COLLIDING
Anything may pass through this brush or patch, whether its players, zombies, grenades or bullets, there will be no collision whatsoever.
This does not effect umbra baking and navmesh generation.